Sometimes I wonder at the uniqueness of each individual's face. It just amazes me how everybody, no matter how numerous we have, has a different face. With the exception of twins and non-familial coincidental twins.
From an artist's point of view, lots of small details and imperfections and things like that. Many bodies have basically the same proportions - your hand is around the same length as the distance between your chin and eyebrows, eyes are an eyeball length apart, nose stops approximately halfway between the corner of the eyes and the chin.
But you probably have one eye slightly higher than the other, one eyebrow that is shaped different, or your nose might turn up ever-so-slightly. Your cheeks might show more "chub" when you smile compared to the next person, even though the weight is similar. I think this is why painted or drawn portraits sometimes don't quite capture someone's face, even though it seems horribly close and is obviously a picture of the subject - and weirdly, why simple caricatures seem to do such a good job. The folks that do them have trained themselves to notice and highlight these sorts of differences.
As a sidenote, I still find it amazing that the human brain is so quick to pick up such things, especially considering it takes training and practice to reproduce on paper.
Practically speaking, every face has to communicate, what, 20 bits of information, to avoid common doppelgängers via the birthday paradox? Probably even more. Very interesting.
People have proposed leveraging this to generate unique avatars, since you can just pick some random parameters for the arrangement of a stick figure face and it will probably look unique.
Openface (probably state-of-the-art currently? If I haven't missed anything) uses a 128-dimensional unit hypersphere to represent a face identity. They have likely optimized this, so it's the dimensionality that works well in practice, not too large, not too small. It's perhaps a bit bigger than it has to be, but it cannot be too big either because that would very likely lead to overfitting.
So that's 128*sizeof(float) = 512bytes = 4096 bits of information.
Kinda like an RSA private key :).
But you probably have one eye slightly higher than the other, one eyebrow that is shaped different, or your nose might turn up ever-so-slightly. Your cheeks might show more "chub" when you smile compared to the next person, even though the weight is similar. I think this is why painted or drawn portraits sometimes don't quite capture someone's face, even though it seems horribly close and is obviously a picture of the subject - and weirdly, why simple caricatures seem to do such a good job. The folks that do them have trained themselves to notice and highlight these sorts of differences.
As a sidenote, I still find it amazing that the human brain is so quick to pick up such things, especially considering it takes training and practice to reproduce on paper.